1. Field of the Invention
This invention relates to a speaker apparatus capable of suppressing a standing wave in the cabinet of the speaker apparatus.
2. Description of the Related Art
As is well known, speaker units are acoustic transducers for emitting sound waves by vibrating diaphragms in response to electric signals (hereinafter called aural signals) carrying acoustic information. Such a speaker unit is rarely employed alone and it is usually fitted to a baffle plate in order to improve sound reproduction and used as a speaker apparatus.
Supposing the size of the baffle plate to which the speaker unit is fitted is unlimited, the interference of the sound wave emitted from across the diaphragm could be prevented completely. However, the speaker unit is actually fitted to a baffle plate of finite size.
The cabinet used in such a speaker is the one embodied therein. A baffle plate of finite size is generally used for one wall surface of a sealed-up or partially opened acoustic box.
FIG. 12 shows an example of a conventional speaker aparatus S showing one speaker unit fitted to a rectangular sealed-up cabinet.
As shown in FIG. 12, a speaker unit 102 is fitted to a baffle plate 101a of finite size formed on one side of a sealed-up cabinet 101. The diaphragm of the speaker unit 102 is driven longitudinally by an aural signal supplied from an input terminal (not shown), whereby sound waves corresponding to the aural signal are emitted from the surface side (the external space side) of the diaphragm so as to effect acoustic reproduction.
As the internal space of the cabinet 101 is a finite space of a cube in this case, the sound wave emitted from the back (on the internal space side) of the diaphragm into the internal space of the cabinet is reflected from a back plate 101b opposite to the baffle plate 101a or a base plate 101d opposite to a top plate 101c and caused to produce standing waves with the opposing wall surface positions as nodes.
The standing wave produced between the wall surfaces at this time includes a higher mode corresponding to a resonance frequency n times (n=positive integer) as great as the lowest resonance mode in addition to a standing wave (the lowest resonance mode) having a wavelength equivalent to what is substantially twice as great as the distance between the wall surfaces.
FIG. 12A shows the lowest resonance modes 103 and 104 out of the standing waves respectively produced between the top plate 101c and the base plate 101d and between the baffle plate 101a and the back plate 101b in the cabinet 101, and FIG. 12B shows higher modes 103a and 104a respectively having resonance frequencies twice as great as the lowest resonance modes out of the standing waves thus produced, that is, having a wavelength half as short as the lowest resonance mode.
These standing waves produced within the cabinet 101 function as those which impair the movement of the diaphragm of the speaker unit 102, which constitutes a primary factor of deteriorating the quality of the sound reproduced by the speaker apparatus S.
Consequently, various contrivances have heretofore been made to ease the standing waves produced in a cabinet as much as possible as shown by examples of speaker apparatus in FIGS. 13A to 13C.
More specifically, the sound pressures of the standing waves have been attenuated by mounting an acoustical material 105 such as glass wool on the inner wall surface of the cabinet 101 as shown in FIG. 13A. The standing waves within the cabinet 101 have also been suppressed as much as possible by mounting a Helmholtz resonator 106 exhibiting resonance at a particular frequency on the inside of the cabinet 101 as shown in FIG. 13B or otherwise using a cabinet 107 having an irregular shape in place of the cubic cabinet 101 so as to eliminate the parallel wall surfaces as shown in FIG. 13C.
In order to satisfactorily attenuate the sound pressures of the standing waves produced in the cabinet 101 by mounting the acoustical material 105 on the inner wall surface of the cabinet 101, however, a considerable amount of acoustical material 105 needs using and as this results in increasing the acoustic resistance, the lower register would be absorbed too.
Though the helmholtz resonator 106 acts on a standing wave at the specific wavelength generated in the cabinet, for example, because it has the effect of absorbing sound at a single resonance frequency, it will not effectively act on a standing wave having any other wavelength.
Moreover, the use of the cabinet 107 having an irregular shape tends to make the speaker apparatus complicated in structure and costly and furthermore to restrict its designing.